The general procedure of drilling an oil or gas well includes drilling a borehole using a drilling fluid. Subsequent to drilling the borehole, casing is run into the well, preparatory to placing cement slurry in the annulus between the outside of the casing and the borehole wall. In order to obtain a good cementing job, it is necessary to displace substantially all of the drilling fluid or mud in the annulus with cement. This necessity arises from the fact that undisplaced mud and filter cake become the source of unsuccessful cement jobs since drilling fluids and cements are usually incompatible. Thus, most oil-based muds will either cause instant setting of the cement or act as a retarder of the cement that can adversely affect the strength. On the other hand, most cement slurries will flocculate and thicken most oil-based muds. As the cement is pumped out of the bottom of the casing and up the annulus, it may form flow channels through blocking sections of flocculated drilling mud. In addition, undisplaced filter cake can prevent cement from bonding to the formation and may become the source of flow channels. Additionally, the cement may shrink upon curing which also may contribute to flow channels.
U.S. Pat. No. 5,382,290 to Shell Oil Co. teaches that a water-in-oil emulsion drilling fluid can be converted into oil mud-cement slurry for use in oil well cementing procedures. Also, a universal fluid can be prepared using the water-in-oil emulsion drilling fluid by treating with a hydraulic material so that it will lay down a filter cake during drilling, which can be triggered to set into a hard cement and bond to the formation after the above oil mud-cement slurry has been placed in the borehole at the conclusion of drilling. Good zonal isolation can be achieved if the filter cake hardens, permanently bonds to the formation face and provides hydraulic sealing.
U.S. Pat. No. 5,464,060 assigned to Shell Oil Co. discloses a composition for use in drilling and cementing a well, thus avoiding removal of the drilling fluid, since the composition can be employed for both functions. The “universal drilling fluid” comprises the product of a drilling mud admixed with a hydraulic material which is suitable for drilling a borehole and laying down a settable filter cake on the walls of said borehole; and an activator mixable with or contacting the filter cake, the activator being functional to cause the filter cake to harden and set up. A preferred way of applying the activator is to conduct a normal cement job with a cement or mud-concrete slurry which carries the activator. The activator may also be admixed with a mud, a spotting fluid, or a pill, and the resulting fluid may be spotted or circulated through the annulus prior to cementing. The activator is subsequently filtered (diffused) through the filter cake and causes it to set hard.
The advantages realized by the Shell invention include the following: (1) a universal fluid is functionally and rheologically suitable as a drilling fluid; (2) the settable filter cake laid down by the universal fluid hardens to a relatively high compressive strength, for example, about 3,500 psi; (3) improved zonal isolation is achieved by the settable filter cake which bonds to the formation and the cementing medium; (4) the bond between the hardened filter cake and the cementing medium is very strong; and (5) it is not necessary either to displace mud or to remove the mud filter cake when a universal fluid is used as a drilling fluid in a well.
Achieving good solution viscosity is important for any fluid pumped down a wellbore; the fluid must have sufficiently high viscosity at low shear rates to suspend solids, yet have low enough viscosity at high shear rates to prevent excessive pump pressures. Managing the rheology of the fluid is critical for well control and to increase drilling rates. Additionally, important time constraints are desired, where it may take 4 to 6 hours to pump a drilling fluid into a very deep well bore, but then the drilling fluid must gel and thicken quickly. Contrarily, waiting 24 or more hours for Portland cement to gel is expensive and runs the risk that within the set time conditions may change in the well that presents additional obstacles to sealing a well.
U.S. Pat. Nos. 7,343,974 and 7,696,133 to Shell Oil co. disclose a composition for strengthening a bore hole, comprising vinyl ester of a C9 to C11 versatic acid, at least one di- or tri-functional acrylate or methacrylate monomer, peroxide initiator, and unsaturated styrenic block copolymer, namely Kraton® D triblock copolymers. Weighting agents like barite (barium sulfate) are incorporated in the composition.
U.S. Pat. No. 7,530,396 to Halliburton Energy Services, Inc. discloses a self-repairing cement composition comprising a cementious material such as Portland cement, an elastic material comprising a polar group that bonds to the cementious material, and water. The elastic material may be butadiene, acrylonitrile copolymers, ethylene and acrylic comonomers. Attached to the elastic material is a polar group such as a carboxylate group having the formula —COOR. Once the cement composition sets, if the surrounding structure cracks or fails, various fluids may seep into the cracks and result in zonal communication. As temperature in the well increases, as a result of fluids flowing from deep strata, the elastic material within the cement composition softens and may even phase change to a liquid and self-repair the crack.
Accessing low margin, highly fractured, and other challenging reservoirs has become increasingly difficult using traditional cement materials. A reactive sealant that has a low solution viscosity, good compressive strength and mechanical properties, such as ductility, and which can be controllably set in a desired zone would allow greater access to such challenging wells. Despite the advantages of such a reactive sealant, there also exists a need in the oilfield for materials that are capable of sealing any post-cure fractures or cracks, for instance through swelling in the presence of a suitable fluid. Additionally, it is well known that hydraulic cements do not perform well in the presence of oil-based mud. A resealable, reactive material that possesses improved retention of mechanical properties in the presence of at least 20 wt. % oil based mud contamination (i.e., at least about 400 psi compressive strength) would therefore be highly desirable, as it may eliminate the need for both hole clean-out and the use of spacer fluids.